A feeding arm usually comprises a kind of a filling station at one end to which the various starting materials such as glass fibers, sand, resin, catalyst as well as further optional filler materials and additives are supplied. The conveyance of said starting materials is performed through pipes of different diameter along the feeding arm to the so-called head of the feeding arm at the other end. There, the continuously conveyed glass fibers (strands of glass fibers, glass fiber yarns) are cut to the respectively desired length with the help of a cutting device, are mixed with the further starting materials and discharged radially (spun off) from where they are centrifuged in a drum-like rotating matrix (mould). The feeding arm moves in an alternating reciprocating manner back and forth in the axial direction of the mould. In this way a pipe with the desired diameter and the desired wall thickness is gradually built up.
Thereafter, the feeding arm is pulled out of the mould and introduced into a new free mould, whereas the previously produced pipe such as a sewage pipe for example is cured in the mould and is thereafter removed from the mould.
Such a method is known as the HOBAS method for the production of centrifuged glass fiber reinforced plastic pipes.
The cutting device consists of three rollers which are arranged in a mutually axially parallel way. A first and second roller are used as conveying rollers. They are both provided with a smooth surface, rotate in opposite directions and their surfaces roll off on one another, whereby they simultaneously convey the strands of glass fibers which are guided along the roller surfaces.
A third roller, a so-called cutter block, is arranged behind the first and second roller. This roller is provided on its surface with a plurality of knives which are arranged spaced from one another in the axial direction of the roller. The cutter block rotates against the direction of the second roller, with the knives rolling off on the surface of the second roller (meaning that they touch the same).
The strands of glass fibers which are conveyed with the help of the first and second roller (conveying rollers) are thereafter guided between the second and third roller and cut to a corresponding length according to the distance between the knives on the cutter block.
The cut glass fibers are thereafter mixed with the further starting materials and discharged from the feeding arm in the manner as described above.
A feeding arm with the aforementioned cutting device has principally proven its worth in practical operations. It has been noticed, however, that the cutter block in particular can heat up to temperatures between 50 and 60° C., particularly during continuous operation. This heating leads to an increased wear of the knives. It was further observed that chemicals will deposit on the surface of the conveying rollers with which the glass fibers are coated for the purpose of better adherence in the matrix material of the pipe to be produced. This, in turn, can lead to increased wear by friction of the surfaces of the conveying rollers and consequently to an adverse grip of the fibers.
The invention is based on the object of providing a feeding arm of the kind mentioned above in which the described increase of temperature in the zone of the cutting device is at least reduced in order to avoid the aforementioned disadvantages.
One possibility would be the exchange of the predominantly metallic rollers against such made of plastic or hardwood which comprise a particularly lower heat conductivity. These rollers, however, comprise a considerably reduced mechanical strength and would therefore have to be exchanged more frequently. This would lead to undesirable standstill periods of the production plant.
Another possibility would be the cooling of the rollers by means of cooling air. This also entails a number of disadvantages: Cooling air would have to be supplied separately. In this respect, an additional cooling air conduit along the feeding arm would be necessary. As a result of the large number of conveying lines in the feeding arm for the numerous starting materials the provision of a further conveying line would lead to a space problem. Moreover, the cooling air would have to be distributed over the entire area of the roller surfaces. This would also lead to considerable space problems. Finally, the cooling air would have to be blown with a respective flow speed onto the rollers, thus making the guidance of the strands of glass fibers and the cut glass fibers uncontrollable in the zone of the cutting device.